Composite materials used in high performance applications are typically being prepared from polyacrylonitrile (PAN)-based carbon fibers and a thermoset matrix material, such as an epoxy. Although such materials exhibit excellent strength and stiffness properties, they are generally limited to moderate operational temperatures (e.g., less than 177.degree. C.) and are relatively brittle (i.e., low impact strength). These limitations are mostly due to the thermoset matrix or resin system. Consequently, much interest has been generated in the development of better matrix materials. Much of this interest has been directed toward thermoplastic matrix or resin systems.
Many thermoplastic resins have significantly higher use temperatures and are tougher (i.e., higher impact strength) than the thermosets (i.e., epoxies) currently used. In addition, most thermoplastic resins are solvent resistant, and have thermal and environmental stability suitable for high performance applications, such as aircraft structures. Also, since thermoplastics consist of molecules that are physically bonded together as opposed to thermosets which consist of chemically bonded molecules, thermoplastics can be reformed. This facilitates repairs to composite structures. Consequently, the use of thermoplastic matrix materials in high performance applications, such as advanced aerospace systems, is anticipated.
One common problem with thermoplastic matrices is that the adhesion of these materials to carbon fibers is typically weaker than that of thermosetting materials. Good fiber-matrix adhesion is necessary in order to produce composites with desirable mechanical properties. If the fiber-matrix adhesion is poor, the composite will fail at the fiber-matrix interface, thus reducing the shear strength and other mechanical properties of the composite. Standard surface treatments of carbon fibers, which are designed for thermoset resins, generally result in little improvement in carbon fiber-thermoplastic matrix adhesion. Since no chemical reaction occurs during the fabrication of thermoplastic matrix composites, the likelihood of forming covalent chemical bonds between the fiber and the thermoplastic matrix material is greatly diminished. The formation of chemical bonds at the interface has been shown to be a significant factor in improving the interfacial bond strength in many composite systems.
Very little information has been published regarding the adhesion of carbon fibers to thermoplastic matrix materials. Thermoplastic matrices are, in general, new to the composites industry. In fact, only two major groups of researchers have published results of studies concerning the adhesion of carbon fibers to thermoplastic matrices. T. A. DeVilbiss and J. P. Wrightman, "Surface Characterization In Composite And Titanium Bonding: Carbon Fiber Surface Treatments For Improved Adhesion To Thermoplastic Polymers," Final Report to NASA-Langley Research Center, Grant No. NAG-1-343, September 1987, and W. D. Bascom, "Interfacial Adhesion Of Carbon Fibers," NASA Contractor Report 178306, Contract NAS1-17918, August 1987.
DeVilbiss and Wrightman studied the adhesion of PAN-based carbon fibers to polysulfone, polycarbonate, and polyetherimide thermoplastic matrices. They used Hercules AU4 and AS4, Dexter Hysol XAU and XAS, and Union Carbide T-300U and T-300S carbon fibers in their studies. The letter "U" in these fiber designations indicates that the fibers were not commercially surface treated. The letter "S" designated that they were commercially surface treated. DeVilbiss and Wrightman determined the shear strength for both the commercially treated and untreated carbon fibers. In addition, they conducted one anodization treatment in 0.5M sulfuric acid and one in 0.5M sodium hydroxide on each type of untreated carbon fiber. Both treatments were conducted at 6 volts for 15 minutes.
For all these composite systems, they reported shear strengths in the range of 14 to 27 MPa (2.0 to 3.9 ksi), which is less than half that of most carbon/epoxy composites. In all three matrix materials, the adhesion was the best for either the sulfuric acid or sodium hydroxide anodized AU4 carbon fiber. The failure location (i.e., the matrix, the interface, or the fiber) was not given.
Bascom et al studied the adhesion of three different PAN-based carbon fibers, Hercules AS4 and AS1, and Grafil XAS, to several different thermoplastic matrices using the embedded single fiber test. Also see: W. D. Bascom, K. J. Yon, R. M. Jensen, and L. Cordner, "The Adhesion Of Carbon Fibers To Thermoset And Thermoplastic Polymers," Conference: Chemistry and Properties of High Performance Composites: Designed Especially for Chemists, West Point, N.Y., October 1988; and W. D. Bascom, "Surface And Interfacial Properties Of Carbon Fibers," NASA-CR-182890, Progress Report NAG-1-706, Oct. 1, 1987-Apr. 15, 1988. Bascom et al determined that the XAS fibers demonstrated better adhesion to these thermoplastics than the AS fibers because of the lower surface basicity of the XAS fibers relative to the AS4 fibers. The AS fibers are more graphitic, and thus, they have a more basic fiber surface than XAS fibers. Water is strongly adsorbed onto these highly basic, graphitic regions present on the AS fibers. Apparently, this adsorbed water inhibits the adsorption of thermoplastic matrices. For a polar matrix system, such as epoxy, this situation is not a problem, since the water is easily displaced by the polar epoxy polymer.
A very limited amount of information has been published concerning the adhesion of carbon fibers to crystalline thermoplastics. Polyetheretherketone (PEEK) is a relatively new, partially amorphous/partially crystalline (semicrystalline) engineering thermoplastic, whose service temperature is above 200.degree. C. PEEK is produced by ICI Corporation, and the majority of the information concerning the fiber-matrix adhesion of PEEK composites is proprietary. However, the open literature does contain a few reports concerning the adhesion of PEEK to various fibers. S. Hamdan and J. R. G. Evans, "The Surface Treatment And Adhesion Bonding Of Polyetheretherketone. Part I. Adhesive Joint Strength," Journal of Adhesion Science and Technology, Vol. 1, No. 4 (1987) pp. 281-289, and J. A. Peacock, B. Fife, E. Nield, and R. A. Crick, "Examination Of The Morphology Of Aromatic Polymer Composite (APC-2) Using An Etching Technique," Composite Interfaces, H. Ishida and J. L. Koenig, Eds., Elsevier Publishing Co., Inc. (1986) pp. 299-305.
Hamdan and Evans reported that the fiber-matrix adhesion of 20 percent fiber volume glass/PEEK composites was improved by chromate etching the PEEK for 30 minutes at 50.degree. C. and by plasma etching the PEEK in oxygen for 15 minutes. Their conclusions were based on lap shear strength tests. Peacock et al used an etching technique to observe the matrix morphology in Hercules AS4/PEEK composites. They determined that their AS4/PEEK unidirectional composites displayed excellent short beam shear strength (viz., 105 MPa or 15.2 ksi) due to the fact that nucleation from the AS4 fibers dominated the morphology. Discrete spherulitic growth occurred at the AS4 surface before initiation started in the matrix. Thus, a high degree of intimacy between AS4 and PEEK was achieved. This nucleation that takes place on the AS4 surface is believed to be stress induced. Crystal growth was oriented perpendicular to the AS4 surface.
Generally, after a carbon fiber is commercially surface treated, a matrix compatible sizing (e.g., a thin layer of epoxy resin) is applied to the fiber surface. The sizing protects the fibers from being damaged when handled, and also helps preserve the surface functional groups. Bascom reported that a 0.1 weight percent application of a phenoxy sizing (PKHC, Union Carbide) on the AS4 surface slightly improved the adhesion of AS4/polycarbonate composites. The embedded single fiber test was used to quantify the fiber-matrix adhesion. No explanation was offered for the observed adhesion improvement.
Recently, titanium-based and zirconium-based coupling agents for carbon fiber/thermoplastic matrices have been developed. S. Monte and G. Sugarman, Ken-React Reference Manual, Kenrich Petrochemicals, Bayonne, N.J. (1987). For example, organotitanate coupling agents have been shown to improve the flexural modulus of carbon fiber/polyetheretherketone (PEEK) and carbon fiber/acrylonitrile-butadiene-styrene (ABS) composites. Ibid. PEEK and ABS are thermoplastic matrix materials. This adhesion improvement, as taught by Monte and Sugarman, is due to the direct reaction of the hydroxyl groups on the carbon fiber surface with the metal (Ti or Zr)-oxygen bond on the coupling agent. The chemical formulas for several specific titanium-based and zirconium-based coupling agents are as follows: ##STR1##
Accordingly, there is a continuing and ongoing need to explore the adhesion between carbon fibers and thermoplastic matrix materials.